APU Question

n57flyguy

n57flyguy

Well-Known Member
Alright, I was reading my handy G4 flight training manual and stumbled upon alot of talk about the APU. Since I dont know all to much about jet systems, what is the APU system (not for the G4, but that would help to) and what does it do? I know its a broad Question but your insights will help. Thanks.
 
It's a small jet engine that runs a generator and provides pressurized air (mostly for the a/c systems).
 
n57flyguy said:
Alright, I was reading my handy G4 flight training manual and stumbled upon alot of talk about the APU. Since I dont know all to much about jet systems, what is the APU system (not for the G4, but that would help to) and what does it do? I know its a broad Question but your insights will help. Thanks.
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Just a hunch, but I think that quote in your signature is from Wolfgang Langewiesche, author of Stick and Rudder!
 
Mavmb said:
Just a hunch, but I think that quote in your signature is from Wolfgang Langewiesche, author of Stick and Rudder!
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Yes, didnt want to mispell his name so I just left it (to lazzzzzzy) But I have read some of the book and that sentence stood out. I'll change it soon...
 
APU is short for Auxulary Power Unit. Some aircraft it can only be used on the ground. Larger jets need bleed air (air from the engine compressor section) to start their main engines, they use the APU bleed air for this. The APU is started with an electrical motor using the ship's battery. Aircraft that have electrical starters for their main engines, it is better to have electricity supplied by the APU than drain the batteries, or it may not be possible to start with just the battries as there isn't enough charge.

I know in the EMB-120 (30pax turboprop) often you can use the APU to run the packs (presurisation and air conditioning) and turn off the engine bleeds all together to get a little more power on the take off run.

Here's an interesting fact, on the ATR they use a prop lock on one engine and use it as a make shift 'APU'

Hope that answers some of your questions,
 
Snow said:
APU is short for Auxulary Power Unit. Some aircraft it can only be used on the ground. Larger jets need bleed air (air from the engine compressor section) to start their main engines, they use the APU bleed air for this. The APU is started with an electrical motor using the ship's battery. Aircraft that have electrical starters for their main engines, it is better to have electricity supplied by the APU than drain the batteries, or it may not be possible to start with just the battries as there isn't enough charge.

I know in the EMB-120 (30pax turboprop) often you can use the APU to run the packs (presurisation and air conditioning) and turn off the engine bleeds all together to get a little more power on the take off run.

Here's an interesting fact, on the ATR they use a prop lock on one engine and use it as a make shift 'APU'

Hope that answers some of your questions,
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Yes it did, thank you very much for your time. I have heard that Bleed air is used for Anti ice on the wings and nose, correct me if Im wrong.
 
n57flyguy said:
Yes it did, thank you very much for your time. I have heard that Bleed air is used for Anti ice on the wings and nose, correct me if Im wrong.
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Bleed air from the engine is used for anti ice on the wings and engine inlets, on jets.
 
lnewby said:
Bleed air from the engine is used for anti ice on the wings and engine inlets, on jets.
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Yes thank you very much. The big pic.... APU is a small jet engine the uses the main battery to fire itself up so the main engines don't drain the batteries or on larger jets, they use the bleed air from the APU to fire em up. Then the APU will start the main engines with either the bleed air form it (APU) or just the power supplied by it.

Another broad question, how does the APU provide AC and Presurisation?
Thanks for your time.
 
n57flyguy said:
Another broad question, how does the APU provide AC and Presurisation?
Thanks for your time.
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My understanding is that it provides the electricity to run the packs. I believe the APU only provides electricity and air, no "other" features... It just provides those to the systems on the rest of the airplane when there's no engine running (or whenever needed)
 
Chris_Ford said:
My understanding is that it provides the electricity to run the packs. I believe the APU only provides electricity and air, no "other" features... It just provides those to the systems on the rest of the airplane when there's no engine running (or whenever needed)
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Correct - EXCEPT that on the planes I'm familiar with the packs run off air (although I'm sure they need electricity as well) they don't heat, cool or pressurize without bleed air from something.
 
Chris_Ford said:
My understanding is that it provides the electricity to run the packs. I believe the APU only provides electricity and air, no "other" features...
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Some do, some don't. Some APUs provide Hydraulic pressure as well. It depends on the model.

The APU in the CH-53 and the C-141 don't use electricity for starting. They use a hydraulic motor powered by pressure from the accumulator. You get three chances to start on the CH-53 and I'm told you only got one chance on the C-141. Then you have to rebuild the pressure with the hand pump.
 
n57flyguy said:
Another broad question, how does the APU provide AC and Presurisation?
Thanks for your time.
Click to expand...

Most if not all APUs provide bleed air to run the packs. With the APU running, you can do just about anything systems wise but for taxi & take off. But it uses a lot less fuel than running an engine too. Such as it's good to keep the airplane cold on the ramp on a hot day but without having to run the engine. Also a bit quieter than the main engines as well, and less of a ground hazzard.
 
USMCmech said:
Some do, some don't. Some APUs provide Hydraulic pressure as well. It depends on the model.

The APU in the CH-53 and the C-141 don't use electricity for starting. They use a hydraulic motor powered by pressure from the accumulator. You get three chances to start on the CH-53 and I'm told you only got one chance on the C-141. Then you have to rebuild the pressure with the hand pump.
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On an HH-60 its 1, unless of course you have external power, but it's fun having the new guy blow out the accumulator then have to pump it back up to 3500 again :)
 
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